The most hotly debated crusade (and if you ask me, the only one to provide amusement along with the massive amounts of depressing) is the Fourth. Those who participated intended to sail to Egypt and proceed from there to Jerusalem, which they would then conquer. Things, shall we say, did not go as planned.

Not at all where they were headed. Somebody fire their navigator.["La Conquête de Constantinople," Geoffrey de Villehardouin, 13th century][Source: Wikimedia Commons]

As they would have to cross much of the Mediterranean, the leaders of the crusade negotiated with Venice for a fleet of ships to carry them. Unfortunately, they both over-estimated the total number of crusaders and under-estimated the number that would choose to go through Venice rather than taking an alternate route. As a result, when they arrived in 1202, they found themselves 21,000 fewer in number than expected and massively in debt to Venice. The Venetians agreed to remit part of the debt if the crusaders would help them re-conquer the city of Zara. As a result of this attack, the Pope excommunicated the entire expedition and demanded they continue on to Jerusalem. This, however, was not to be as they were persuaded by Alexios, son of the recently deposed Byzantine Emperor, to return him to Constantinople. He told them the Byzantines would welcome him with open arms and he would pay off the crusaders’ debt to the Venetians. Once again, the expected did not happen, and the expedition ended up besieging and conquering Constantinople in 1203, putting Alexios and his father on the throne, and still not getting paid.

Yeah, they stole a bunch of stuff.["Horses of Basilica San Marco," possibly 4th century][photographed by Morn, Source: Wikimedia Commons]

Eventually the crusaders, French and Venetian alike, starving and probably incredibly frustrated, besieged and conquered the city again, this time for themselves.* The Latin Empire they set up lasted only 53 years before the Byzantines retook both city and empire. Instead of conquering Jerusalem, the crusaders ended up sacking two Christian cities, getting excommunicated, and setting up an empire they did not have the resources to run. * There is significant scholarly debate as to whether this was the Venetians' aim all along. While I am strongly inclined to think it was not, that is a topic for another time.